1. Field of the Invention
The present invention relates to power conversion circuits operating in a switching mode and more particularly to circuit arrangements for the protection of semiconductor components from voltage spikes which may occur on the power supply rail.
2. Description of the Prior Art
A recurrent problem in power circuits operating in a switching mode and directly or indirectly connected through a rectifier and a low-pass filter to the power distribution network is the protection of the circuits from transient voltage peaks or "spikes" which may occur on the power supply network.
According to a widely used technique, protection elements, having a Zener-type electrical characteristic and capable of absorbing the energy of a spike thus limiting the maximum voltage "seen" by the. circuit to be protected, are employed. These protection elements may be voltage-dependent resistors, avalanche diodes, Zener diodes or special integrated structures having a similar behaviour, such as for example the so-called TRANSIL structure (TRANSIL is a trade name employed by the Thomson Company), as it is well known to a skilled technician.
A typical example of a circuit which is so directly connected to the power supply network is the so-called FLYBACK converter, the circuit diagram of which is depicted in FIG. 1. The two transistors M1 and M2, functionally representing two analog switches, during a normal operation of the circuit are driven "in-phase", alternately ON and OFF, at a certain frequency and with a certain "duty-cycle". The two driving signals, respectively Ph1 and Ph2, which in the case of the circuit shown are essentially coincident signals (Ph1=Ph2), may be generated by a Pulse Width Modulation (PWM) control circuit, in accordance with a technique familiar to the skilled technician, which utilizes a feedback network which, in the case of the converter depicted in FIG. 1, includes comparator means for comparing a signal replica of the output voltage Vo of the circuit with a preset reference voltage in order to generate an error signal for the PWM control loop. The "duty-cycle" of the two driving signals determines the controlled output voltage Vo.
During normal operating conditions, the transistor M1 is subjected to a maximum voltage about equal to the rectified supply voltage Vin during OFF periods, while the transistor M2 is subjected to a maximum voltage about equal to the output voltage Vo. Similarly the recirculation diode D1 is subjected to a reverse voltage about equal to Vin while the output diode D2 is subjected to a reverse voltage about equal to Vo. When a spike presents itself on the input supply rail (Vin+spike), the following conditions may occur:
(a) with both transistors M1 and M2 conducting, the diode D1 is subjected to the entire spike voltage and the energy contained in the spike causes a remarkable increase of the current through the inductance L, which is connected to ground through the transistor M2 and to the input rail through the transistor M1. In this case the current is limited only by the impedance of the inductor L subjected to the entire voltage, and it should be remembered that in many instances the impedance of the inductor L may become extremely low because of a possible saturation of the core;
(b) with both transistors M1 and M2 nonconducting, the transistor M1 is subjected to the entire spike voltage while the transistor M2 remains subjected to a normal operating voltage about equal to the value of the output voltage Vo.
As shown in FIG. 1, the conventionally adopted solution for reducing the voltage and the energy "seen" by the circuit in case of spikes on the supply network employes a voltage limiting element capable of withstanding the dissipation of the spike's energy, represented in FIG. 1 by a Zener diode Zs, connected across the input terminals of the converter. The energy dissipating element Zs, which may also be something else than a Zener diode, such as an avalanche diode, a voltage-dependent resistor, a TRANSIL or in any case an integrated structure having a voltage/current characteristic showing a definite breakdown voltage, as said before, limits the maximum voltage seen by the circuit to be protected to about the value of the intrinsic breakdown voltage of the element (Zs). This known solution has the drawback of requiring the use of a voltage limiting element capable of withstanding in practice the dissipation of the entire energy associated with the spike.